Multi-modality Study of the Compositional and Mechanical Implications of Hypomineralization in a Rabbit Model of Osteomalacia

摘要

Osteomalacia is characterized by hypomineralization of the bone associated with increased water content. In this work we evaluate the hypotheses that 1) 3D solid-state magnetic resonance imaging (MRI) of ³¹P (SSI-PH) and ¹H (SSI-WATER) of cortical bone can quantify the key characteristics of osteomalacia induced by low-phosphate diet; 2) return to normophosphatemic diet (NO) results in recovery of these indices to normal levels. Twenty female five-week old rabbits were divided into four groups. Five animals were fed a normal diet for 8 weeks (NOI); five a hypophosphatemic diet (0.09%) for the same period to induce osteomalacia (HYI). To examine the effect of recovery from hypophosphatemia additional five animals received a hypophosphatemic diet for 8 weeks, after which they were returned to normal diet for 6 weeks (HYII). Finally, five animals received a normal diet for the entire 14 weeks (NOII). The NOI and HYI animals were sacrificed after 8 weeks, the NOII and HYII groups after 14 weeks. Cortical bone was extracted from the left and right tibiae of all the animals. Water content was measured by SSI-WATER and by a previously reported spectroscopic proton-deuteron nuclear magnetic resonance (NMR) exchange technique (NMR-WATER), phosphorus content by SSI-PH. All MRI and NMR experiments were performed on a 9.4 Tesla spectroscopy/micro-imaging system. Degree of mineralization of bone (DMB) was measured by μ-CT and elastic-modulus and ultimate-strength by 3-point bending.The following parameters were lower in the hypophosphatemic group: phosphorus content measured by SSI-PH (9.5±0.4 versus 11.1±0.3 wt %, p<0.0001), ash content (63.9±1.7 versus 65.4 ±1.1 wt %, p=0.05), ultimate-strength, (96.3±16.0 versus 130.7±6.4 N/mm², p=0.001), and DMB (1115 ± 28 versus 1176±24 mg/cm³, p=0.003); SSI-WATER: 16.1±1.5 versus 14.4±1.1 wt%, p=0.04; NMR-WATER: 19.0±0.6 versus 17.4±1.2 wt%, p=0.01. Return to a normophosphatemic diet reduced or eliminated these differences (SSI-PH: 9.5±0.9 versus 10.6±0.8 wt %, p= 0.04; DMB: 1124±31 versus 1137±10 mg/cm³, p=0.2; US: 95.6±18.6 versus 103.9±7.5 N/mm², p=0.2; SSI-WATER: SSI-WATER: 12.4±0.6 versus 12.2±0.3 wt%, p=0.3) indicating recovery of the mineral density close to normal levels. Phosphorus content measured by SSI-PH was significantly correlated with DMB measured by μ-CT (r²=0.47, p=0.001) as well as with ultimate-strength (r²=0.54, p=0.0004). The results show that the methods presented have potential for in situ assessment of mineralization and water, both critical to the bone’s mechanical behavior.